The present invention relates to a screw compressor system and operating methods thereof wherein a plurality of screw compressors can be operated in parallel, in particular, a screw compressor system and operating methods thereof suitable for performing capacity control in response to an amount of consumption of compressed gas generated in the screw compressor system.
In relation to a compressed air production equipment comprising a plurality of screw compressors, in order to minimize the consumed power, use of one compressor whose rotational speed is variable in combination with a plurality of compressors each having a fixed rotational speed is disclosed in JP-A-2000-161237. In the compressed air production equipment disclosed in this publication, the rotational speed of the variable-speed compressor is controlled preferentially and then the plurality of fixed-speed compressors are operated and stopped by turn-back control or rotary control.
JP-A-4-159491 discloses that one screw compressor is used with the switching cycle between full load operation and no-load operation being changed so as to prevent wear and tear of parts due to frequent on/off-operations.
Since the compressed air production equipment disclosed in JP-A-2000-161237 include the variable-speed compressor, the equipment has an advantage that the equipment is highly efficient throughout a wide range of load factor, which is represented by the consumed gas volume relative to the rated discharged gas volume of a compressor, and the power consumption can be reduced. However, when the discharged gas volume of the equipment is increased, the capacity of the variable-speed compressor cannot but be increased accordingly. But, such a large-capacity variable-speed compressor is expensive. This brings about an in-convenience that the production cost of the compressed air production equipment is increased.
The screw compressor disclosed in JP-A-4-159491 is premised on being used alone. JP-A-4-159491 does not consider that a plurality of compressors are operated at once. Between such a screw compressor system for producing compressed gas and a demander, in general, there are passage parts such as filters, gas storage towers, and piping, wherein the passage resistance varies in accordance with the gas velocity flowing therein. In other words, the pressure loss in piping or the like reduces as the load factor reduces. Conventionally, the discharge pressure of such a compressor is set by taking into consideration with the pressure loss at the maximum flow rate. However, in order that the compressor may not consume excessive power, when the pressure loss reduces, it is desirable to set a suitable discharge pressure of the compressor accordingly.
The present invention has been made in view of the inconveniences of the above prior arts and its object is to reduce shaft power in a screw compressor system comprising a plurality of load/no-load operation type screw compressors and realize power-saving operation.
A screw compressor system of the present invention to attain the above object is characterized by comprising control means for determining the number of screw compressors to be operated in accordance with the compressed gas consumption in a demander, putting, in the screw compressors to be operated, all second screw compressors other than one first screw compressor in load operation, and changing discharge pressure of each of the first and second screw compressors in accordance with the load factor of said first screw compressor.
In this characteristic feature, it is preferable that when the load factor of the first screw compressor has reduced, the control means reduces compressor discharge pressure upon load operation start and compressor discharge pressure upon no-load operation start respectively to be less than a rated compressor discharge pressure upon load operation start and a rated compressor discharge pressure upon no-load operation start predetermined in relation to the first screw compressor. Besides, it is preferable that when the reduced compressor discharge pressure upon load operation start is beyond a predetermined lower limit, the control means sets the compressor discharge pressure upon load operation start to that lower limit.
Further, it is preferable that when period from the load operation to subsequent load operation of the first screw compressor is beyond a predetermined time range, the control means lowers the compressor discharge pressure upon no-load operation start of the first screw compressor than the predetermined rated compressor discharge pressure upon no-load operation start.
Another screw compressor system of the present invention to attain the above object is characterized by comprising a plurality of compressors and in that a parent controller obtains a load factor on the basis of a cycle time measured by a timer, determines the number of screw compressors to be operated in accordance with that load factor, in the screw compressors determined to be operated, puts one screw compressor in load operation, controls the remaining one to repeat load operation and no-load operation, as for this one screw compressor, obtains a load factor on the basis of a cycle time newly measured by the timer, and changes the discharge pressure measured by discharge pressure measuring means in accordance with that load factor. It is preferable that the parent controller controls one screw compressor so that the discharge pressure measured by the discharge pressure measuring means is lowered when the load factor reduces.
An operating method of a screw compressor system of the present invention to attain the above object is characterized in that a load factor is obtained from the cycle time of load operation and no-load operation obtained by operating all screw compressors, the number of screw compressors to be operated is determined on the basis of that load factor, one of the screw compressors to be operated is operated to repeat load operation and no-load operation, the remaining screw compressors of the screw compressors to be operated are put in load operation, in relation to the screw compressor repeating load operation and no-load operation, a cycle time is newly measured to obtain a load factor, and the discharge pressure of the screw compressor repeating load operation and no-load operation is changed in accordance with that load factor.
In this characteristic feature, it is preferable that the discharge pressure upon load operation start and the discharge pressure upon no-load operation start of the screw compressor repeating load operation and no-load operation is lowered as the load factor reduces and further, when the discharge pressure upon load operation start of the screw compressor repeating load operation and no-load operation has reached a predetermined lower limit pressure, it is preferable that the discharge pressure upon load operation start is set at that lower limit value and the discharge pressure upon no-load operation start is changed.
Preferably, the discharge pressure is controlled by a parent controller provided for one of a plurality of compressors included in the screw compressor system and the parent controller controls child controllers respectively provided for the remaining screw compressors.